Bacterial-host interactions can be considered as multifactorial equations in which bacterial virulence factors on one side and host immune defense mechanisms on the other determine the ultimate outcome of infection. Using Bordetella bronchiseptica and Bordetella pertussis as models for chronic and acute respiratory infection, respectively, the investigators hope to identify and characterize bacterial factors involved in overcoming immune defenses which operate at various sites in the respiratory tract. The investigators have developed natural-host animal models and a variety of in vitro, in vivo and ex vivo techniques for characterizing adherence, colonization and the host immune response to infection. They have extensive experience manipulating the Bordetella virulence regulon and have constructed a collection of genetically defined phase-locked, deletion, and ectopic expression mutants for probing the role of virulence genes and virulence gene regulation in vivo. Their goal is to combine these resources and apply them towards an effort to understand the function of bacterial virulence factors. The research plan begins with an examination of requirements for colonization of the nasal cavity. This event is largely unexplored and it occurs with remarkable efficiency in their B. bronchiseptica animal models. Bordetella genes required for colonization of this site will be identified and characterized. They will then examine requirements for tracheal colonization with emphasis on filamentous hemagglutinin (FHA). A possible role for FHA in determining host specificity will be explored using chimeric Bordetella strains expressing heterologous fhaB loci. Finally, although the BvgAS virulence control system has historically been viewed as mediating a transition between two distinct phases, the authors' preliminary data suggest that the system may be significantly more complex than previously thought. The Principal Investigator plans to investigate a recently discovered Bvg intermediate phase gene and test the hypothesis that this phase is essential for transmission.